1. Field of the Invention
The present invention relates to a high-amperage current sensor for use in automotive electrical systems. More particularly, the invention relates to an automobile current sensor which is operatively unaffected by changes in ambient temperature.
2. Description of the Prior Art
Demands placed upon the automotive industry by consumers and legislators to produce more fuel efficient vehicles have resulted in the development of a new breed of automotive sensor electronics. One particular sensor is designed to monitor steady state current drain of the car battery. This current sensor functions to monitor current loading of a specific segment of the automotive electrical system and develop an analog or digital measure of the current for use by a remote sensor computer. Data read from the current sensor is needed to determine adjustments required for improving fuel economy and other aspects of vehicle performance. In one mass-produced automobile, for example, sensor data is used to determine when to remove voltage from the alternator in order to reduce engine drag during periods of low current drain. The result of such action is a cumulative savings in fuel cost and an improvement in gas mileage.
Unlike signal applications utilizing "microammeters," automotive current sensors are required to measure high-amperage currents often approaching 200 amperes. Present systems for measuring these high current levels generally utilize a Hall-effect device mounted near the cable through which the current is being passed. These Hall-effect sensors function to develop an output voltage proportional to the magnetic field surrounding the cable.
Although Hall-effect sensors provide good electrical isolation, they are known to have many disadvantages. For example, a Hall-effect current sensor will not develop an output voltage distinguishable from system noise for magnetic flux densities below a specified magnitude. Thus, to obtain finer current measurements, it is often necessary to loop the current line one or more times through or around the sensor. This is often impractical in an automobile where space is at a premium. Additionally, the output off a Hall-effect current sensor is proportional not only to the magnetic field detected around the current line, but also to the excitation current applied through the Hall semiconductor material. Thus, for example, a 5% change in excitation current will produce the same output voltage as a 5% change in the magnetic field surrounding the current line. Since the two changes are indistinguishable, Hall-effect current sensors generally must be equipped with highly regulated current sources. This simply adds to the complexity and cost of the device.
Additionally, the output voltage of Hall-effect current sensors is affected by changes in ambient temperature. This is due to the relatively high temperature drift characteristic of the Hall semiconductor material. Specifically, a 1.degree. C. change in ambient temperature changes output voltage in these sensors by approximately 0.1%. Thus, in applications where ambient temperature is expected to vary by more than a few degrees, it is generally necessary to compensate for the resulting variations in output voltage. This requires compensation circuitry which further adds to the complexity and cost of the device.
The need to accurately measure high-amperage current levels is even greater in electric automobiles currently being developed to reduce consumption of fossil fuels. These vehicles will almost certainly employ precision control systems, such as current injection systems, to drive multi-phase electric motors. In such systems, it is desirable to have current sensors capable of flat frequency response up to 100,000 Hertz. It is also generally desirable in these applications to have sensor phase delay of less than 5% to frequencies above 40,000 Hertz. The frequency response of Hall-effect sensors, however, is appreciably attenuated at frequencies above approximately 30,000 Hertz. As the current approaches and surpasses this frequency, excess phase delay is also introduced.